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1.
The remanent magnetization of igneous and sedimentary rocks, if not changed by heating or by alteration of magnetic minerals, keep the information about the intensity of that magnetic field in which initial magnetization took place.It has been determined that the dependence of anhysteretic remanent magnetization of such a rock on d.c. magnetic field permits us to find the paleointensity. A method of investigation of rock specimens by means of such remagnetization is suggested and applied to determining the paleointensity for a series of Permo-Triassic rocks. 相似文献
2.
J.L. Roy 《Physics of the Earth and Planetary Interiors》1977,13(4):319-324
Polyphase magnetizations are not uncommon in old rocks. To obtain reliable paleointensities, these magnetizations need to be recognized and separated so that the paleointensity determination can be derived from one of the phases of magnetization. Some of the techniques used to detect and separate (and sometimes isolate) the different phases are described by means of a few examples. Special attention is given to the chemical remanent magnetization which can be found in both sedimentary and igneous rocks. It appears that a sedimentary rock near an igneous contact is the preferferred specimen for reliable paleointensity determinations. 相似文献
3.
A. L. Piskarev Ch. Heunemann A. A. Makar’ev A. M. Makar’eva V. Bachtadse M. Aleksyutin 《Izvestiya Physics of the Solid Earth》2009,45(2):150-166
A combined study of magnetic parameters of basalt and andesite samples is performed in the framework of geological investigations of the Franz Josef Land at the paleomagnetic laboratory of Munich University. The study included the determination of the coercivity, saturation magnetization, Curie points, natural remanent magnetization (NRM), and magnetic susceptibility and the examination of ferromagnetic minerals with a microscope. Data on the chemical composition of rocks are obtained for all samples, and radiological ages are determined for the majority of rocks.Thermomagnetic curves of samples are subdivided into four types depending on the composition of ferromagnetic NRM carriers.The data obtained point to multiple changes in the predominant composition of igneous rocks. Each stage of magmatism is characterized by a specific type of the ferromagnetic component in the rocks and, therefore, magnetomineralogical investigations can be used for differentiation and correlation of the igneous rocks. 相似文献
4.
V. A. Rashidov O. V. Pilipenko V. V. Petrova 《Izvestiya Physics of the Solid Earth》2016,52(4):550-571
The rock magnetic properties of the samples of dredged rocks composing the submarine volcanic edifices within the Sea-of-Okhotsk slope of the northern part of the Kuril Island Arc are studied. The measurements of the standard rock magnetic parameters, thermomagnetic analysis, petrographical studies, and microprobe investigations have been carried out. The magnetization of the studied rocks is mainly carried by the pseudo-single domain and multidomain titanomagnetite and low-Ti titanomagnetite grains. The high values of the natural remanent magnetization are due to the pseudo-single-domain structure of the titanomagnetite grains, whereas the high values of magnetic susceptibility are associated with the high concentration of ferrimagnetic grains. The highest Curie points are observed in the titanomagnetite grains of the igneous rocks composing the edifices of the Smirnov, Edelshtein, and 1.4 submarine volcanoes. 相似文献
5.
The opinions vary on the secondary remagnetization mechanisms of carbonate rocks. The prevalent interpretations of magnetic remanences are either chemical in origin that invoked the large-scale migration of orogenic fluids. or thermoviscous magnetization involved burial diagenetic processes. The paleomagnetic study carried out in northwestern part of Henan Province (north of the Qinling orogenic belt) reveals that 90% of carbonate rocks have suffered from the secondary remagnetization. On the basis of rock magnetic experiments, the assemblage of the magnetic minerals, remanent carriers and its formation process, and the possible mechanisms of remagnetization in these carbonate rocks are discussed. 相似文献
6.
7.
O. V. Pilipenko V. M. Trubikhin H. Abrahamsen J. -P. Buylaert 《Izvestiya Physics of the Solid Earth》2010,46(12):1052-1063
During revisiting the Upper Pleistocene Pekla loess-soil section located on the Sea of Azov coast of the Taman Peninsula,
its lower 6 m were continuously sampled, which led to an increase in the age range from ∼50 to 400 ka. The detailed rock magnetic
study of the structure, grain-size, and concentrations of magnetic mineral (natural remanent magnetization (NRM) carrier)
in the collected rock samples revealed regular changes in rock magnetic characteristics along the section and their correlation
with climatic fluctuations. Magnetite and hematite both deposited during the transport of sedimentary material and formed
during pedogenesis, which involved the entire section to a varying extent, represent the main magnetic minerals in the examined
rocks. Automorphic paleosoils that were formed during warm and humid periods corresponding to odd stages of the MIS scale
are characterized by elevated concentrations of magnetic mineral (NRM, magnetic susceptibility (K
lf), saturation isothermal remanent magnetization (SIRM), and anhysteresis (ideal) remanent magnetization (ARM)) parameters
and share of superparamagnetic particles (up to 80%, according to elevated values of the frequency-dependent magnetic susceptibility
K
td) as well as by lowered rigidity parameter (B
cr) and grain size (ARM/K parameter). Such changes in the paleosoils may be explained by the occurrence of newly formed fine-grained magnetite particles
close in size to its superparamagnetic and single-domain varieties due to the activation of bio/geochemical processes during
warm stages. The growth of the above-mentioned rock magnetic parameters in automorphic soils may be considered as serving
a quantitative criterion for defining the boundary between warm and cold periods even in poorly developed soils. 相似文献
8.
Takesi Nagata 《Pure and Applied Geophysics》1970,78(1):100-109
Summary Characteristics of remanent magnetization of an igneous rock under a uniaxial compression are divided into those of the soft component and those of the hard component. The soft component of remanent magnetization irreversibly decreases with an increase of compression () regardless whether the axis of is parallel or parpendicular to the direction of magnetization. The hard component changes reversibly with : the magnetic intensity of hard component increases with when the axis of is perpendicular to the magnetization, whereas it decreases with when the axis of is parallel to the magnetization.Theoretically, the irreversible change of the soft component is attributable to the irreversible movement of 90° domain walls which results in the pressure demagnetization effect, while the reversible change of the hard component is due to the reversible rotation of spontaneous magnetization within those domains which are so tightly fixed by large effective anisotropy energy that the induced magnetoelastic energy cannot drive their 90° walls.In natural remanent magnetization of igneous rocks, the soft component corresponds to either one or all of isothermal remanent magnetization, viscous remanent magnetization and piezo-remanent magnetization, whereas the hard component is mostly due to thermo-remanent magnetization. 相似文献
9.
The interpretation of the Jarrafa magnetic and gravity highs, NW Libyan offshore, suggests that it may be caused by a body of high-density and high magnetization. Analysis of their power spectra indicates two groups of sources at: (1) 2.7 km depth, probably related to the igneous rocks, some of which were penetrated in the JA-1 borehole, (2) 5 km depth, corresponding to the top of the causative body and (3) 10 km depth, probably referring to the local basement depth. The boundary analysis derived from applied horizontal gradient to both gravity and magnetic data reveals lineaments many of which can be related to geological structures (grabens, horsts and faults).The poor correlation between pseudogravity fields for induced magnetization and observed gravity fields strongly suggests that the causative structure has a remanent magnetization (D = −16°, I = 23°) of Early Cretaceous age, fitting with the opening of the Neo Tethys 3 Ocean.Three-dimensional interpretation techniques indicate that the magnetic source of the Jarrafa magnetic anomaly has a magnetization intensity of 0.46 A/m, which is required to simulate the amplitude of the observed magnetic anomaly. The magnetic model shows that it has a base level at 15 km.The history of the area combined with the analysis and interpretation of the gravity and magnetic data suggests that: (1) the source of the Jarrafa anomaly is a mafic igneous rock and it may have formed during an Early Cretaceous extensional phase and (2) the Jarrafa basin was left-laterally sheared along the WNW Hercynian North Graben Fault Zone, during its reactivation in the Early Cretaceous. 相似文献
10.
A. Kapička 《Studia Geophysica et Geodaetica》1992,36(2):168-176
Summary With the decreasing magnitude of the initial remanent condition of rocks, their pressure demagnetization gradually changes to pressure remanent magnetic polarization under elastic deformation. In both cases the physical cause of these changes are the irreversible changes of the domain structure of ferrimagnetic minerals. Under directional pressure the natural remanent magnetic polarization is affected namely by the generation of a relatively little stable pressure remanent magnetic polarization. With regard to paleomagnetic research, the essential thing is that secondary magnetization combined with possible elastic deformations of rocks in the Earth's crust can be eliminated relatively easily by magnetic cleaning. 相似文献
11.
Sudhir K. Dutt 《Pure and Applied Geophysics》1964,59(1):84-92
Summary The rock specimens found to have natural remanent magnetization of abnormally high intensity, have been generally from hill sides or tops, or from ridges high up from the local surroundings. A field of several hundred oersteds has been found sufficient to produce in some of the artificially demagnetized specimens an isothermal magnetization of the same order as the abnormal natural ones, and this magnetization has shown a similar degree of stability as the natural one. Variations in the direction and magnitude of the natural magnetic vector have been found over a distance of a few centimetres. Laboratory tests indicate a normal chemical composition for the specimens. The lightning discharge seems to be the plausible cause of abnormally high magnetization of rocks, which is generally isothermal and might have originated in the recent past, but the magnetization is sometimes complicated probably by the thermal effect of the discharge. 相似文献
12.
Summary The relation between remanent magnetization and volumetric strain for gabbro samples stressed in uniaxial compression inside a near zero-field -metal shield has been examined. For samples with an induced IRM parallel to the axis of compression, remanent magnetization decreased linearly up to the onset of dilatancy. As increased stress produced additional dilatancy, the variation of remanent magnetization became nonlinear, and the stress dependence continually decreased until the rock failed. Stress cycling with the peak stress augmented for each cycle produced a continuous decrease in the zero stress value of the IRM although an appreciable amount of recovery was observed during unloading. When the sample was loaded in constant stress increments after the onset of dilatancy and held for several minutes at each level, time-dependent variations in remanent magnetization coincided with time-dependent increases in inelastic volumetric strain. In general as the inelastic creep rate increases, the rate of change in remanent magnetization increases. These results suggest that dilatancy related effects of the intensity of rock magnetization should be observed in magnetic rocks in epicentral regions prior to earthquakes and may serve as both long- and short-term precursors. 相似文献
13.
Herbert Henkel 《Pure and Applied Geophysics》1976,114(2):235-249
Summary Over the shield region of Northern Sweden, the Geological Survey runs a low altitude (30 m) aeromagnetic survey and regional gravity surveys cover almost the same areas. The production of detailed geological maps would be almost impossible without these geophysical measurements. To enhance their interpretation, a study of the appropriate petrophysical properties was started by measurements of density, susceptibility and remanence on all rock specimens collected by field geologists. Up to now more than 30,000 specimens have been measured and almost as many in situ susceptibility determinations have been made. About 10% of the samples are oriented. The information obtained is useful not only for the direct interpretation of geophysical surveys but also reflects the effects of various geological processes acting during and after the formation of the rocks considered.In precambrian rocks, density is obviously closely correlated to mineral composition and thus to chemical composition. This is demonstrated for igneous rocks by the correlation trends between density and SiO2-content and the CM/AF-index.Susceptibility mainly reflects the magnetite content of rocks. As magnetite is an accessory mineral it is seldom considered by geologists. However, a closer study of the magnetic susceptibility of rocks reveals that its extreme complexity reflects the effects of primary and secondary geological processes. The susceptibility spectrum of a certain rock may prove to be a useful classification tool — at least regarding intermediate and basic igneous rocks.The combination of the physical parameters density and susceptibility in 2-dimensional frequency distributions seems to be a promising approach to the understanding of certain petrological processes and makes possible the delineation of local or regional secondary processes. Magmatic differentiation and serpentinization can be demonstrated in this manner.Remanent magnetization plays a secondary role in the majority of precambrian rocks. Still there are places where the natural remanence dominates and shows pronounced directions deviating from today's magnetic field. So far we have only one case where a follow up by demagnetization has been attempted, but intensified paleomagnetic research should most certainly add to our understanding of precambrian geology.Combination of susceptibility and remanance shows some characteristic correlations for highly remanent rocks, reflecting mainly exosolution phenomena among magnetic opaques and grain size distributions. Even these features might be used as diagnostic or classifying tools. They also explain some of the diversity of susceptibility spectra. 相似文献
14.
G. A. Pospelova M. V. Anikovich J. F. Hoffecker M. Kadzialko-Hofmokl 《Izvestiya Physics of the Solid Earth》2007,43(12):1031-1046
A magnetic method is proposed for reconstructing the paleoclimate existing at the formation time of rocks of the Paleolithic Kostenki-12 site section. Results obtained from an experimental study of scalar magnetic characteristics (SMCs) of the rocks (magnetic susceptibility, anhysteretic remanent magnetization, and saturation isothermal remanent magnetization) are presented. These results indicate complex along-section SMC behavior distinguished by alternating larger and smaller values of the parameters. The magnetic minerals of the section are mainly represented by pseudo-single-domain magnetite grains. The size of the magnetite grains in the pseudo-single-domain range varies along the section, increasing in its lower half. The SMCs values plotted on different scales for the upper and lower halves of the section clearly revealed 12 magnetic zones (MZs). Comparison of the MZs with palynological zones obtained in a parallel section showed, on the whole, a convergence of data: MZs with larger (smaller) SMC values correlate with thermomers (cryomers). Comparison of along-section SMC variations with the GRIP (Greenland Ice Core Project) curve of the time dependence of paleotemperatures confirmed or refined the ages of lithologic horizons. As a result, it is established that constraints on paleoclimatic changes in the rock formation time can be gained from SMC variations if the main carriers of magnetization in rocks are pseudo-single-domain magnetite grains even with magnetic grain size varying along the section. 相似文献
15.
断层岩,尤其是断层泥的磁性异常近年来被研究人员广泛关注,但关于其磁性异常的形成原因尚没有统一的解释．海原断裂是青藏高原东北缘一条重要的走滑断裂,前期研究发现海原断裂带景泰段出露有数十米至上百米的断层岩,是理想的研究材料．本研究选取海原断裂带景泰段老虎山山前一个断层岩剖面作为研究对象,拟通过测量断层岩的磁化率（χ）、非磁滞剩磁（ARM）、饱和等温剩磁（SIRM）、等温剩磁（IRM）以及磁化率随温度变化曲线（χ-T曲线）等磁学参数并结合粒度、碳含量、X射线衍射（XRD）等分析方法来探究海原断裂带老虎山段不同颜色断层岩的磁性特征及其形成机制．磁学研究显示黑色、红色及杂色断层泥相较于围岩和破碎带显示了低磁性,尤其是黑色断层泥,其磁化率值均小于10×10~(-8 )m~3·kg~(-1)．碳含量及矿物相分析结果指示黑色断层泥与断裂带附近石炭系煤层具有相似的矿物相组成,结合相似的χ-T曲线推断石炭系煤层为黑色断层泥的母岩．石炭系煤层经断层活动卷入断层,在断层强烈剪切摩擦作用下不断细化,形成伊利石等黏土矿物,并促使一部分顺磁性含铁硅酸盐矿物或其他含铁矿物发生化学变化形成亚铁磁性矿物,使得黑色断层泥的磁化率较其母岩石炭系煤层有一定升高．通过黑色断层泥的铁磁性磁化率结合χ-T曲线计算获得断层泥所经历的最高温度约为420℃,不超过450℃．老虎山段厚层碳质断层泥的存在为该地区发现的浅层蠕滑现象提供了一种解释． 相似文献
16.
V. A. Rashidov O. V. Pilipenko V. M. Ladygin 《Journal of Volcanology and Seismology》2014,8(3):168-182
We studied the petrophysical and magnetic properties of dredged rocks that compose the edifices of five active submarine volcanoes in the Pacific Ocean, viz., the Fukujin, Esmeralda, Kovachi, and Simbo at island arcs, and Ile des Cendres in a marginal sea. We measured the standard petrophysical and magnetic characteristics and performed three kinds of thermomagnetic analysis as well as an electron-probe analysis. Comparative analysis of magnetic properties in rocks showed that they are strongly differentiated by the value of natural remanent magnetization and of magnetic susceptibility. The greatest values are shown by the Esmeralda aphyric basaltic andesites, the lowest occur in the Ile des Cendres xenoliths. The principal carriers of magnetization in the rocks are grains of unaltered and/or oxidized titanomagnetite with varying domain structures. All the samples are magnetically isotropic. 相似文献
17.
Experiments combining different cleaning and analytical techniques indicate that multiphase magnetizations may be quite common. However, these may not be recognized because of limited experimental work. Alternating field (AF) cleaning is often the only treatment applied to igneous and metamorphic rocks; thermal and/or AF cleanings are usually the only treatments applied to sedimentary rocks. In many instances, AF and thermal treatments are limited to 100 mT and 550°C respectively. Investigations based on such limited cleaning often fail to detect some of the phases of magnetization contained in the rock. Failure to detect one or more phases means that valuable data are not recovered and the whole magnetic history of the rock has not been unfolded. Most importantly, the undetected phase might be the initial so that a secondary magnetization can easily be mistaken for the initial with an erroneous interpretation as a result. It is therefore imperative to recognize all phases of magnetizations and it possible to separate them.Procedures that have been used to recognize and unravel multiphase magnetizations are described. These procedures make use of chemical, thermal and AF cleaning treatments, two-stage demagnetization, vector analysis, slicing of specimens and vector diagrams. The combination used depends on the rock studied. For example, it is found that AF followed by thermal treatment can be very useful for the study of igneous rocks; chemical leaching is by far the most effective cleaning technique for the study of red beds. A three-phase model describing the magnetizations process of certain red beds is proposed. The slicing of specimens is used to explain intermediate directions with respect to field reversals. It is shown that graphical representation by vector diagrams can greatly facilitate the interpretation of the results. The examples show that, although a statistically well defined magnetization may be obtained after limited cleaning, it cannot be assumed to be the initial. One must ascertain that another magnetization has not remained undetected. This necessitates detailed and extensive experimental work using and devising new analytical procedures in an attempt to unfold the whole magnetic history of the rock. It is noted that tentative polar paths constructed from results obtained after inadequate experimental work cannot be up-graded by adding more data points of doubtful or unproven quality. The evolutionary process of polar paths is dependent upon increasing the reliability of palaeomagnetic results. 相似文献
18.
Our rock magnetic analysis of core Ph05 from the West Philippine Sea demonstrates that the core preserves a strong, stable
remanent magnetization and meets the magnetic mineral criteria for relative paleointensity (RPI) analyses. The magnetic minerals
in the sequence are dominated by pseudosingle-domain magnetite, and the concentration of magnetic minerals is at the same
scale. Both the conventional normalizing method and the pseudo-Thellier method were used in conjunction with the examination
of the rock magnetic properties and natural remanent magnetization. Susceptibility (χ), anhysteretic remnant magnetization (ARM) and saturation isothermal remnant magnetization (SIRM) were used as the natural
remanent magnetization normalizer. However, coherence analysis indicated that only ARM is more suitable for paleointensity
reconstruction. The age model of core is established based on oxygen isotope data and AMS14C data, which is consistent with the age model estimated from RPI records. The relative paleointensity data provide a continuous
record of the intensity variation during the last 200 ka, which correlates well with the global references RPI stacks. Several
prominent low paleointensity values are identified and are correlated to the main RPI minima in the SINT-200 record, suggesting
that the sediments have recorded the real changes of geomagnetic field.
Supported by National Natural Science Foundation of China (Grant No. 90411014) and Pilot Project of the Knowledge Innovation
Program of Chinese Academy of Sciences (No. KZCX2-YW-211) 相似文献
19.
Two techniques, namely alternating-field demagnetization and thermal demagnetization, are widely being used for determining the stability of magnetization of a rock specimen. Recently a faster and simpler technique known as low-field hysteresis loop and memory-phenomenon test has been developed for determining the stability of magnetization directions of igneous rocks. In this paper the results of this new technique, after applying it to about 1000 specimens obtained from 250 oriented rock samples collected from 42 sites of Deccan Trap basalts from Mount Girnar (21°30′N; 70°30′E) and Mount Pavagarh (22°30′N; 73°30′E), India, are presented.The agreement between the mean natural remanent magnetization directions determined by this procedure and those computed after alternating-field demagnetization has been found to be very good. All the specimens from Mount Girnar and 94% of Mount Pavagarh specimens showed a stable line without any memory in a field ≈ 10 Oe. This indicates that the rocks from these two localities are highly stable and are most suitable ones for the determination of a precise palaeomagnetic direction. 相似文献
20.
V. P. Shcherbakov N. K. Sycheva S. K. Gribov 《Izvestiya Physics of the Solid Earth》2017,53(5):645-657
The results of the Thellier–Coe experiments on paleointensity determination on the samples which contain chemical remanent magnetization (CRM) created by thermal annealing of titanomagnetites are reported. The results of the experiments are compared with the theoretical notions. For this purpose, Monte Carlo simulation of the process of CRM acquisition in the system of single-domain interacting particles was carried out; the paleointensity determination method based on the Thellier–Coe procedure was modeled; and the degree of paleointensity underestimation was quantitatively estimated based on the experimental data and on the numerical results. Both the experimental investigations and computer modeling suggest the following main conclusion: all the Arai–Nagata diagrams for CRM in the high-temperature area (in some cases up to the Curie temperature T c) contain a relatively long quasi-linear interval on which it is possible to estimate the slope coefficient k and, therefore, the paleointensity. Hence, if chemical magnetization (or remagnetization) took place in the course of the magnetomineralogical transformations of titanomagnetite- bearing igneous rocks during long-lasting cooling or during repeated heatings, it can lead to incorrect results in determining the intensity of the geomagnetic field in the geological past. 相似文献